In the manufacture of molded records such as conventional audio records or the more recently developed videodisc, a plastic material is molded between a pair of metal discs referred to as stampers. The stampers have defined in their molding surfaces an information track which contains the surface relief pattern representative of information desired to be molded into the molded records. The stampers are the end product of a replication process known generally as matrixing.
The first step in the record making process is to record the program information desired to be molded into the final record on a magnetic tape. The magnetic tape is then used to control a tool which cuts an information track containing the desired programing into a flat disc called a recording substrate. The recording substrate is usually a metal plate which has a layer of material on its surface into which the information track is cut; in the case of audio records, the material is generally made of wax or a synthetic lacquer; in the case of video discs, the material is generally a layer of an electrodeposited metal such as a bright copper.
The recording substrate, having the information defined in its surface, is thereafter replicated in a series of electroforming steps. In the first step, a metal, such as nickel, is electrodeposited on the recorded surface of the recording substrate until a self-sustaining part is obtained. The resulting electroformed part, called a master, is then carefully separared from the recording substrate. The master is then in turn replicated by electroforming a metal such as nickel, on the recorded surface of the master. After a sufficient thickness of metal has been deposited on the master to form a self-sustaining part, the resulting replica, referred to as a mold, is separated from the master. The molds which are obtained are then in turn likewise replicated to produce a series of parts known as stampers. The stampers that are obtained are negative replicas of the starting recording substrate and are used to mold the records, as noted above.
In the matrixing process parts such as the masters and molds are initially formed as replicas and then are themselves used as matrixes that is a part on which a replica is formed. Since the present invention is generally useful in the matrixing process without specific regard to whether the part is actually a recording substrate, master, mold, or stamper, the term matrix will be used hereinafter to refer to a part which is replicated and the term replica will be used hereinafter to refer to a part which is electroformed on the matrix.
One of the major problems encountered in the matrixing process is premature separation of the replicas from the matrixes on which they are electroformed, which can occur at various stages in the matrixing process. For example, premature separation can occur during the electroforming of the metal onto the matrix. This results in the production of defective parts in that the replica will have insufficient thickness, and furthermore, it causes leakage of electrolyte between the replica and the matrix which can result in burnouts of the matrixes and other related problems. Also when the replica has been completely electroformed on the matrix and is being removed from the electroforming apparatus or being transferred to other operations, if a replica prematurely separates at this point, it is often damaged by being dropped or the like. Also, as a result of premature separation, there is often sliding contact between the matrix and the replica which results in either or both the replica and the matrix being scratched and irreparably damaged.
The problem of premature separation is an extremely difficult problem to overcome in that certain required matrixing procedures and the required surface characteristics of the replicas and matrix are in part a cause of the problem. For example, in the matrixing process the matrixes must be passivated so as to prevent the adhesion and possible alloying of the metal which is electrodeposited to form the replica with the metal of the matrix. For this reason, it is highly desirable for most purposes in the electroforming operation, especially when it is desired to separate the replicas from the matrix, to have as passive layer as possible deposited on the matrix. The matrixes and the resulting replicas are also deliberately designed so that they have not undercutting or the like which mechanically interlock the parts since such undercuts would adversely affect the release properties when molding plastic materials. In addition, the outer edges of the matrixes and the center portions of the matrixes are designed to be smooth, almost mirror-like, so as to promote release of the molding materials from the stampers. Even in the recorded areas of the replicas, especially on a video disc, the area is effectively smooth because of the relative closeness and the shallowness of the information track in the program area.
Various attempts have been made to reduce or eliminate premature separation of replicas and matrixes. One such suggestion was to preplate matrixes with the outer edge of matrixes exposed so as to induce plating over and around the outer edge of the matrixes. The plating over the outer edge of the matrix was intended to form a grip to hold the replica in place on the matrix during completion of the matrixing process. This technique has, however, not proved to be satisfactory as it has had limited success in reducing premature separation, has added a separated time-consuming step to the matrixing process and has caused problems in separation of the replicas from the matrix because the extra metal on the edge plating has to be removed either by being cut off or ground off the matrix.
It was also suggested in the prior art to knurl the unrecorded outer edge of the matrixes and possibly also the center of the matrixes so as to improve the grip of the replicas to the matrix. Knurling, however, has not proven to be a satisfactory solution. It was found that the knurling had to be forced into the replicas for a substantial depth in order to be effective in holding a replica onto the matrix. The amount and depth of knurling required resulted in considerable amount of metal being displaced in the knurling process which in turn caused distortion of the information track in the recorded area of the replicas. A further problem encountered with knurling was that the knurling if sufficiently deep enough resulted in deep lines or raised ridges in the outer edge of the molded recorded which was highly undesirable with regard to use of that portion for the lead-in track on the records.
What would be highly advantageous would be a process to reduce or eliminate premature separation of replicas from their matrixes which is simple to perform, and which would not result in any adverse effects on the final molded records.